This invention relates to a process for preparing polymers of alpha methylstyrene. More particularly this invention relates to a suspension polymerization process for producing polymers of alpha methylstyrene, acrylonitrile and styrene in bead form at a high level of monomer conversion, having high molecular weight and low residual monomer content.
Conventional suspension or "bead" polymerization processes are known for the preparation of a variety of polymers such as polystyrene and styrene-acrylonitrile copolymers. These methods generally are characterized by incomplete conversion of the monomers, and consequently the resulting bead polymers are recovered containing varying high levels of residual monomer present. A variety of techniques have been developed to avoid the presence of unreacted monomer in the bead product, such as for example that shown in U.S. Pat. No. 3,288,731 wherein the polymerization reaction mixture is treated with a strong base at a monomer-polymer conversion point of about 75-85 percent to remove the unreacted acrylonitrile prior to isolating the beads. This method produced bead products relatively free of residual acrylonitrile but the low conversions result in lowered yields, a substantial loss of monomers and a concommitant increase in production costs. Alternate approaches include the use of carefully programmed process temperatures to increase the level of conversion followed by steam distillation to remove unreacted monomers such as is taught in U.S. Pat. No. 3,491,071. This latter process, employing styrene, alpha methylstyrene and acrylonitrile, requires the use of programmed process temperatures followed by steam distillation of unreacted monomer (acrylonitrile) prior to 98% conversion. The use of programmed process temperatures and steam distillation of unreacted monomers adds substantially to the costs, and additionally does not avoid the loss of some monomer through incomplete conversion. Further, when applied to terpolymers wherein alpha methylstyrene comprises the largest weight fraction of the monomer mixture, conversion is low and the molecular weight of the resulting product is significantly decreased.
An efficient process for producing high molecular weight polymers from mixtures of alpha methylstyrene, acrylonitrile and styrene having high percentages of alpha-methylstyrene is clearly needed.